It takes time to get the hang of it. If you don’t enjoy pedaling on your mtb don’t bother with the road bike. If you have some nice scenary, enjoyable roads it can provide nice experiences. Also very good for general fitness. I find my mtb to fly when I hop on it from my road bike. On the road bike I enjoy suffering, pushing to my limits, improving my times, chasing cars, etc. On the mtb I just go up and have a blast coming down.
The 54 cannondale should be your size. I’m sure the 56 is too big. You’ll be stretched out. Always look at the effective top tube length. I’m same size as you (portions may vary though). I ride bikes with around 54cm top tubes. Currently a Tarmac with 54,8cm but I’m constantly swapping a 90 and a 100 stem. Haven’t decided yet. Sometimes I miss the sportiness of the 100, sometimes I enjoy the relaxed position that the 90mm provides.
One more thing. Gearing is much higher on the road bike. I have steep grades where I live and I struggled even with the compact chainset and 28 casette. After a season or so you sure get the strength it requires. If you consider yourself a bit weaker a triple might help you to keep your cadence up. Very important aspect. If you’re riding flat just forget what I just wrote.
You don’t want a bike too good if you ride in filthy conditions though. You don’t want that fancy carbon to scratch or break when you fall, which is easy on those skinny slicks (debris, wet).